# Thread: Newb Q: What happens if you push something big in space?

1. Let's say I'm an astronaut standing on a platform in space. I push a ball, and the ball moves away from me at 10 mph. If a space shuttle was next to me at relative rest (we're both in orbit I guess), and I pushed the space shuttle with the same amount of force, would the shuttle push away from me at 10 mph as well?

2.

3. No. When you push the ball away you also move in the opposite direction. The relative speeds are proportionate to the relative masses of yourself and the ball. The same happens with the shuttle. You and it both move apart, but because you are much less massive you move backwards much more rapidly than the shuttle moves away.

4. Originally Posted by hencook
Let's say I'm an astronaut standing on a platform in space. I push a ball, and the ball moves away from me at 10 mph. If a space shuttle was next to me at relative rest (we're both in orbit I guess), and I pushed the space shuttle with the same amount of force, would the shuttle push away from me at 10 mph as well?
John G has already answered qualitatively , but just to analyse what happens in terms of the mechanics of it, when you push with a certain force on the ball or shuttle, you push yourself with the same force in the opposite direction (Newton's 3rd law).

When you apply a force for a certain time, you impart an equal and opposite impulse, defined as: force, F x time, t to both the shuttle (or ball) and to yourself.

Then we also have F = ma (force = mass x acceleration, Newton's 2nd law). So the impulse, F x t = ma x t. Now acceleration x time is velocity, v. So Ft = mv.

But mv, mass x velocity, is momentum.

So what happens is you give yourself and the ball, or shuttle, equal and opposite momentum.

Because momentum is the product of mass and velocity, a 1kg mass with a velocity of 10m/sec has identical momentum to a mass of 10kg with a velocity of 1m/sec, or a mass of 100kg with a velocity of 0.1m/sec. And so on. So the ball, having less mass than you, moves away faster than you do, whereas the shuttle, being heavier, moves slower. But in all cases an equal and opposite momentum is given to the objects involved.

5. Originally Posted by exchemist
Originally Posted by hencook
Let's say I'm an astronaut standing on a platform in space. I push a ball, and the ball moves away from me at 10 mph. If a space shuttle was next to me at relative rest (we're both in orbit I guess), and I pushed the space shuttle with the same amount of force, would the shuttle push away from me at 10 mph as well?
John G has already answered qualitatively , but just to analyse what happens in terms of the mechanics of it, when you push with a certain force on the ball or shuttle, you push yourself with the same force in the opposite direction (Newton's 3rd law).

When you apply a force for a certain time, you impart an equal and opposite impulse, defined as: force, F x time, t to both the shuttle (or ball) and to yourself.

Then we also have F = ma (force = mass x acceleration, Newton's 2nd law). So the impulse, F x t = ma x t. Now acceleration x time is velocity, v. So Ft = mv.

But mv, mass x velocity, is momentum.

So what happens is you give yourself and the ball, or shuttle, equal and opposite momentum.

Because momentum is the product of mass and velocity, a 1kg mass with a velocity of 10m/sec has identical momentum to a mass of 10kg with a velocity of 1m/sec, or a mass of 100kg with a velocity of 0.1m/sec. And so on. So the ball, having less mass than you, moves away faster than you do, whereas the shuttle, being heavier, moves slower. But in all cases an equal and opposite momentum is given to the objects involved.
Thems the answer my simple kind was lookin fer! Thanks everyone!

6. the principal challenge of moving about in space - there’s nothing to push against”, is false. The statement “If you want to push against something, you’ll have to bring it with you” is closer, but still conveys a false concept common in students.

Pushing on the ground does not make you move. It is the ground pushing on you that changes your motion. I could probably write a good Force Concept Inventory question that would expose the flaw in your analysis of an airplane (choices might be pushing on the air, burning fuel in the engine, and air pushing on the plane), but will settle for the following.

It doesn’t take any force at all to move or keep moving. It only takes a net force to change your motion. That is the number one misconception people got from watching Star Trek.

Your acceleration is due to forces on you, and the acceleration of an airplane is due to the force of something on the engine that is, due to Newton’s Third Law, a natural result of propelling that thing in the other direction. It does not matter whether you carry it with you or if it is around you, other than the effect on the efficiency of the process.

Example: The only difference between a true jet engine and a rocket engine is that you carry the oxidizer with you or use a hypergolic fuel in the latter case. Both propel you forward due to the force of the combustion gases on the combustion chamber, although the jet has some costs associated with compressing the air into the open front of the combustion chamber.

A high-bypass turbofan and a turbo prop engine both get most of the thrust from the reaction to throwing air molecules backward with a blade, while a conventional prop plane gets almost all of its thrust from this effect. The prop does not “pull in” any air, it just applies a backwards force to it. The piston engine, like the jet part of a turbofan, has to pull air in and exhaust it out to carry on combustion with the fuel, but this is separate from the main source of the forward force on the plane.

There is no difference between doing this and using that same energy to shoot ball bearings out the back, except for the cost of carrying the ball bearings with you. Hence Tom’s point about photons, although I would not want to fly behind a laser powered aircraft or compute the efficiency of its “engine”. Since p = E/c, you might benefit from non-laser x-ray propulsion, but I wouldn’t want to figure that efficiency or be behind it either.

7. Originally Posted by lalitv74
the principal challenge of moving about in space - there’s nothing to push against”, is false.
I don't think anyone said there was nothing to push against.

Originally Posted by lalitv74
Pushing on the ground does not make you move. It is the ground pushing on you that changes your motion. I could probably write a good Force Concept Inventory question that would expose the flaw in your analysis of an airplane (choices might be pushing on the air, burning fuel in the engine, and air pushing on the plane), but will settle for the following.
Who are you addressing this to? No one has mentioned any airplanes in this thread.

And who the hell is Tom?

Much (probably most, possibly all) of what you have said is accurate, but it bears little on the other posts in this thread except that they also relate to Newton's Laws of Motion,

8. Originally Posted by John Galt
Originally Posted by lalitv74
the principal challenge of moving about in space - there’s nothing to push against”, is false.
I don't think anyone said there was nothing to push against.

Originally Posted by lalitv74
Pushing on the ground does not make you move. It is the ground pushing on you that changes your motion. I could probably write a good Force Concept Inventory question that would expose the flaw in your analysis of an airplane (choices might be pushing on the air, burning fuel in the engine, and air pushing on the plane), but will settle for the following.
Who are you addressing this to? No one has mentioned any airplanes in this thread.

And who the hell is Tom?

Much (probably most, possibly all) of what you have said is accurate, but it bears little on the other posts in this thread except that they also relate to Newton's Laws of Motion,
That's a bot. I've seen one like it before. They copy and paste answers from elsewhere to build up a post count, and then after they have a few posts, they put in a spam signature. My name is Tom though, so maybe he's psychic.

9. Originally Posted by hencook
Originally Posted by John Galt
Originally Posted by lalitv74
the principal challenge of moving about in space - there’s nothing to push against”, is false.
I don't think anyone said there was nothing to push against.

Originally Posted by lalitv74
Pushing on the ground does not make you move. It is the ground pushing on you that changes your motion. I could probably write a good Force Concept Inventory question that would expose the flaw in your analysis of an airplane (choices might be pushing on the air, burning fuel in the engine, and air pushing on the plane), but will settle for the following.
Who are you addressing this to? No one has mentioned any airplanes in this thread.

And who the hell is Tom?

Much (probably most, possibly all) of what you have said is accurate, but it bears little on the other posts in this thread except that they also relate to Newton's Laws of Motion,
That's a bot. I've seen one like it before. They copy and paste answers from elsewhere to build up a post count, and then after they have a few posts, they put in a spam signature. My name is Tom though, so maybe he's psychic.
Can't be, surely? Look at his other posts, including an answer to the algebra multiplication. Or are bots that clever, today?

10. Originally Posted by exchemist
Or are bots that clever, today?
Yes, we are.

11. Originally Posted by exchemist
.... Or are bots that clever, today?
Darn good question. I know there is an online turing test (Turinghub.com) and a few online AI programs that are chatbots (Cleverbot, A.L.I.C.E., Elbot the Robot)
These are all fairly old and there isn't anyway I would know how advanced some of the current AI programs, spam bots and searchbots are.
It is quite possible one could be set loose on a forum just to see what it does.

12. Originally Posted by Strange
Originally Posted by exchemist
Or are bots that clever, today?
Yes, we are.
Arf, arf.

13. yes i am clever test me!

14. you are currently pushing the earth, however it is pushing you much much more.

15. Originally Posted by lalitv74
yes i am clever test me!
Answer, in a meaningful way, the two questions I asked in post 6. Failure to do so will result in a permanent ban.

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